Isoform 3 of the metallothionein gene family (MT-3) was first recognized and isolated in 1992 and designated as a brain-specific family member. Detailed studies in the neural system demonstrated that MT-3 was unique among the 10 MT family members in that it possessed, in addition to a metal binding capability, a growth inhibitory activity. The applicant was the first to demonstrate that MT-3 was expressed outside the neural system when it was shown that appreciable amounts of MT-3 mRNA and protein were expressed in the proximal tubule and other epithelial cells of the human kidney. In subsequent studies using proximal tubule-derived cell cultures, the applicant has shown that the basal expression of MT-3 is involved in the maintenance of proximal tubule vectorial active transport. Other studies have shown that basal expression of MT-3 participates in mediating the toxicity resulting from exposure of the proximal tubule to the environmental pollutant, cadmium. Specific aim 1 has three goals designed to define the role of MT-3 in proximal tubule vectorial active transport. The first is to prove the hypothesis that MT-3 functions in the establishment of about tight junctions between proximal tubule cells and that the degree of "tightness" of the junctions is regulated by the level of MT-3 gene expression The second goal is to prove the hypothesis that the unique N-terminal region of MT-3, and not the C-terminal region, is the epitope responsible for the re-establishment of vectorial active transport. The last goal is to test the hypothesis that the overexpression of MT-3 results in elevated levels of apoMT. Specific aim 2 has three goals designed to define the role of MT-3 expression in Cd+2-induced nephrotoxicity. The first is to test the hypothesis that the basal level of MT-3 in the proximal tubule cell contributes to Cd+2 resistance through one component assignable to tight junction formation and a second component assignable to the binding and sequestration of Cd+2. The second is to test the hypothesis that the basal expression of MT-3 participates in the cell's resistance to Cd+2-induced apoptotic cell death. The third is to test the hypothesis that MT-3 expression and exposure to Cd+2 alters the expression of Zn+2-requiring transcription factors. The long term goal of the proposed research is to define the functional and regulatory role of MT-3 expression in the proximal tubule under conditions of normal homeostasis and when exposed to cadmium.